Question

How and when can we use the words "just" and "only" ?

Answer 1

There are various shades of meaning for these.

"Only" can imply something isn't as big as it might be, for instance

"It's only a little spider",

"She was only a checkout operator from a small town"

"Only" and "just" can also mean limited to one idea, or excluding other things; in this sense they are interchangeable, for instance

"I only/just want to be left alone for a while"

"she's only/just jealous"

"I only wanna be with you" (listening to Dusty Springfield sing this is joyful!)

Often we say "just" to mean simply

"I just can't stand him"

"That's just gorgeous!"

Answer 2

The word "just" can mean "very recently" or "only".

The rule to know when "just" meas "very recently" or "only": just+noun=only+noun, but just+verb is not the same as only+verb.